backends/libinput: Turn calibration matrix property into a string
Transporting a QMatrix4x4 over D-Bus is difficult, not really helped by QtDBus. The property doesn't show up in the generated QtDBus interface for example.
Using QMatrix4x4 also introduced subtle but hard to fix bugs due to the nature of how QtDBus serialization worked (or doesn't work, usually.)
Now the property uses QString-based serialization ala the pressure curve property. This is backwards-compatible with existing config values too.
The relevant KCM change: plasma-desktop!2676 (merged)